1. Field of the Invention
The present invention is directed to a signal evaluation apparatus (DSP), and method, for detecting items of information transmitted according to the DS-CDMA (Direct Sequence--Code Division Multiple Access) principle wherein: (a) the items of information, dependent on the time t, of several connectors k are simultaneously transmitted in a frequency channel; and (b) the reception signals (e(t)) received by the signal evaluation apparatus are formed by the convolution of transmission signals (d.sub.k (t)) with individual channel coefficients (h.sub.k (t)) representing transmission channels and with the spread codes (c.sub.k (t)) of the individual connections k.
2. Description of the Prior Art
In the transmission of digital signals from a transmitter apparatus to a receiver apparatus via time-variant channels, (e.g., non-stationary radio channels), there occurs runtime differences of the signal components received over the individual detours of the multipath channel as a result of the multipath propagation. In the signal detection of relatively high bitrates (e.g., in the GSM mobile radiotelephone system), these runtime differences lead to extremely strong and time-variant adjacent symbol interferences (inter-symbol interferences) of the transmitted symbols. Such inter-symbol interferences of signal components can amount to the temporal displacement of several symbols of useful information. In order to still be able to recognize the signal symbols under such influences, adaptive echo equalizers have been used; e.g., the Viterbi algorithm for the rapid finding of an adequate equalization optimum is applied.
An estimate of the complex filter coefficients of the radio channel, which ensues using a channel estimator, is a precondition for the equalization of adjacent symbol interference. For example, it is known from the GSM mobile radiotelephone system that the channel estimator adequately specifies the radio channel by means of an evaluation of a training sequence contained in each radio block. These complex filter coefficients are obtained by means of correlation and evaluate the temporal distribution, caused by the multipath propagation, of the signal components of the reception signals--also called a complex impulse response of the radio channel. Thus, each of these channel coefficients represents the complex integral of the level value of all signal components falling within the evaluation time period of the duration of a symbol. These channel co-efficients are determined by means of standard matching algorithms such as LMS (least mean square) or RLS (recursive least square).
The channel coefficients are supplied to the signal detection, and respectively represent, for the duration of a radio block, a basis of calculation for an adequately reliable detection of the symbols transmitted in this radio block. Since, as a rule, non-stationary radio channels are of concern, the channel coefficients must be adapted per radio block to the changes in the radio channel. Erroneous decisions during adaptation endanger the stability of the signal detection and ultimately lead to errors in detection.
When detecting items of information according to the DS-CDMA principle, additional problems arise. In the radio channel, the information of several connections is transmitted on the same carrier frequency at the same time. The connections are thereby distinguished in a spread code that respectively characterizes each individual connection. Upon reception of the signals from different signal sources, as is typical, for example, in this multipoint-to-point DS-CDMA transmission scenario (i.e., in the uplink of a CDMA receiver apparatus of a base station in mobile radiotelephone systems), the signals spread using different spread codes yet arrive at the reception site at the same time. Apart from the different spread codes, these signals also have different runtimes, are subject to different detour profiles and have different levels that are compensated by means of a relatively exact and quickly reacting power control in order to avoid strong disturbances.
The individual spread codes of the connections are typically orthogonal to one another. The individual signals must be separated from the summation mixture by means of correlation with the individual code. The different runtimes of the radio paths involved, which are to be equated with an imperfect synchronization of the signals, as well as the different channel impulse responses produce mutual interferences (multiple access interference=MAI). The use of matched filters in the receiver apparatus thereby supplies a non-negligible cross-correlation with interfering signal portions of other spread codes.
From the German patent application 195 491 48.3, it is known to additionally take into account the spread codes of additional connections upon reception, and detection, of items of information transmitted according to the CDMA principle. The method used is known as the JD-CDMA method (JD standing for Joint Detection). The separation and detection of the items of information of the individual connections are carried out in one step, by reconstructing the transmission signals according to a global optimization criterion and with knowledge of all the spread codes of the connections of the radio channel. The computing outlay required in a signal processing apparatus for this realization is, however, very large. As such, it is difficult to realize real-time processing while taking many spread codes into account.